This invention relates to the field of fuses, more particularly time delay dual element cartridge fuses capable of interrupting circuits under both overload and short circuit conditions.
Time delay fuses include a short circuit element connected to a spring trigger mechanism to form a fusing assembly, which is then held in an insulated tube and mechanically and electrically connected to opposed end ferrules. A time delay low voltage cartridge fuse of this type is disclosed in U.S. Pat. No. 4,992,770 (Spalding et al.).
The Spalding fuse has a short circuit element anchored to the spring trigger mechanism through a spring loaded bullet member. The bullet member is a cold headed part and extends outwardly from a barrel having a spring therein to bias the bullet away from the short circuit element under overload conditions. A paper cap surrounds the barrel with the closed end of the paper cap covering the end of the barrel opposite the bullet. The paper cap insulates the heater strip, which carries current from one end of the fuse, from contact with the barrel which is made of brass. This arrangement also ensures the current flows through the heater strip and does not bypass through the barrel to the ferrule. The paper cap also isolates the interior of the barrel from arc quenching fillers, such as sand, disposed within the fuse.
In order to ensure that the heater strip, bullet and short circuit element are mechanically and electrically linked, a fusing alloy is disposed at the juncture of the bullet, barrel and heater strip and at the juncture of the bullet and short circuit element. The fusing alloy is a low melting point solder designed to melt when the heat given off by the heater strip and short circuit element from a long term overload condition elevates its temperature to the melting point. Upon assembly of this sub-assembly into the insulating tube, the heater strip is folded over the sides of the insulated barrel and is soldered to a ferrule creating an electrical path therebetween.
In operation, the fuse will open under two types of conditions. If a short circuit is encountered, the heat produced in the short circuit element, which is caused by the passage of excess electric current through the necked portions thereof, causes the short circuit element to melt, opening the circuit across the opposed end ferrules of the fuse. Under long term overload conditions, the electric current flowing through the heater strip and short circuit element generates heat and after a sufficient period of time the heat will cause the fusing alloy to melt. This causes the spring to retract the bullet into the barrel, thus causing the bullet to pull away from the short circuit element, opening the circuit through the fuse.